Modern wind energy installations have rotors with adjustable rotor blades. The pitch angle of the rotor blades can be varied with respect to the airflow by the adjustment. The rotor blades are also adjusted in order to stop the rotor of the wind energy installation. For this purpose, the rotor blades are moved to a so-called wind bearing position. For safety reasons, the wind energy installations must be designed such that the rotor can be brought to rest quickly when required. This is done on the one hand when the wind is excessively strong, but on the other hand also when parts of the wind energy installation are believed to have failed. The rotor blades are adjusted to the safe wind bearing position in the control mode by means of the blade adjustment drive and its control system. However, it cannot be assumed for safe emergency operation that the control system is still completely available. The blade adjustment device should be designed such that the rotor blades can be moved quickly and safely to the wind bearing position even in uncontrolled emergency operation.
It has been shown that, in certain conditions during uncontrolled emergency operation, the rotor blade to be adjusted is adjusted more quickly on the basis of aerodynamic forces than the speed corresponding to that of the actuating motor. The actuating motor is then driven by the rotor blade, and acts as a generator. The current flow through the armature revolves. This so-called feedback current flows through the series winding, thus increasing the magnetization. The induced voltage rises, which can lead to a further increase in the current. Overall, this results in undesirable positive feedback, which can lead to dangerous instabilities. In particular, the load acting on the tower head of the wind energy installation changes, as a result of which oscillations can occur on the tower.
In order to prevent the occurrence of such instabilities during slip operation as well, it is known in the case of actuating motors which are in the form of compound-wound motors for the series winding to be bridged by means of a diode during slip operation. The current which is produced by the actuating motor during slip operation is short-circuited by the diode in such a way that, in a corresponding manner, no more current flows through the series winding, and the magnetization correspondingly decreases. The drag torque of the motor is thereby reduced (EP-A-1 744 444). However, it has been found that, with certain designs, particularly of compound-wound motors, the short-circuiting of the series winding leads to the torque/rotation-speed characteristic having an undesirable profile, to be precise with the increase in the torque with respect to the rotation speed being too flat. This results in the risk of overspeeding, with damage to the adjustment device.